Diaphragm carburetors generally used to supply fuel to two-cycle engines are configured so that fuel fed from a fuel pump is regulated at a fixed pressure by a fuel pressure regulator, and is sent to the intake path. The fuel pressure regulator is equipped with a fuel chamber that stores fuel sent from the fuel pump. A diaphragm that forms one of its walls and a control valve that is interlocked to the motion of the diaphragm opens and closes the fuel chamber inlet. Thus in any position, fuel can be supplied properly to the engine. As a means of feeding fuel chamber fuel to the intake path, the carburetor is equipped with both a main fuel path that leads to a main nozzle that is open to a venturi and an idle fuel path that leads to slow and idle ports that are open to the side of a throttle valve.
However, a diaphragm carburetor has a very low fuel flow rate compared with a float-type carburetor that supplies fuel to a four-cycle engine (e.g., an automobile engine). Therefore, even if the locations and dimensions of the main nozzle, idle port, slow port, and fuel path deviate only slightly, fuel flow rate fluctuation increases so it becomes impossible to supply the proper amount of fuel. Moreover, the diaphragm that operates the inlet valve by sensing the fuel chamber pressure varies in quality, and thus the flexibility or rigidity differ greatly. Therefore, the fuel chamber fuel cannot be regulated at a given fixed pressure.
Therefore, in order to eliminate fuel supply variation caused by quality variation and dimensional deviation in the diaphragm carburetors, a manual adjustment valve is provided for independently controlling the effective areas of the main and idle fuel paths, and a carburetor or engine specialist inspects and adjusts each product before shipment.
However, the adjustment valve comprises a needle-shaped valve that changes, in a non-stepwise manner, the effective area of the fuel path into which it is inserted, a screw that is screwed into the carburetor and moves back and forth while turning a valve, and a head that projects from the carburetor and is used to turn the screw. The adjustment valve can be turned freely, even by users of light vehicles and machines with such a diaphragm-carburetor-equipped engine as the power source. Thus, in an attempt to adjust an out-of-tune engine or to improve engine performance, the user sometimes manually turns the most accessible adjustment valve. The user then frequently over-rotates it, thereby misadjusting the fuel flow rate, which easily worsens the exhaust composition and engine operation. Moreover, if the user rotates both valves, it will affect the entire engine operation.
Thus, in such prior devices manual adjustment valves that independently control the effective areas of the main and idle fuel paths are easily rotatable by a general user, so there is a risk of misadjusting the fuel flow rate, thereby degrading both the exhaust composition and the engine operation.
The present invention is directed at providing a diaphragm carburetor that prevents the degradation of exhaust composition and engine performance by allowing the general user to rotate the adjustment valves only within a limited range.